Forensic - Impression Evidence Unit

Visible shoe/tire marks

Contaminated shoe or tire prints

Impression shoe/tire marks

shoe or tire prints impressed into a material (most common)

Latent shoe/tire marks

prints located by altenative lighting (least common)

How to make a cast in 4 steps

1. setting up a dam

2. stabilizing with spray

3. powder with silicon powder

4. pouring casting mental (dental cement) around the impression

What is used to make a cast in snow?

snow print wax

What is the machine used to collect marks left in dust?

electrostatic dust lifter

5 different features use to analyze marks

thread patterns, width & depth of thread pattern (size), wear pattern, damage or defects, contaminants

What are contaminants?

materials on the shoe/tire that could indicate where a person has been

What is a tool mark?

any impression, cut, gouge, or abrasion caused by a tool when in contact with another object

What are indented took marks?

when a tool contacts an object with enough compressive force, the shape of the tool and its imperfections are left as an impression

What are striated took marks?

when a took contacts an object with lateral force and motion leaving ridges or striations behind

The 3 ways took marks are collected

1. photographed or sketched

2. collecting whole object with took marks

3. casting the impression

What happens when a tool, shoe, or tire found from a suspect is a possible match with a crime scene

test mark suspect’s tool with crime scene mark

4 features analyzed when looking at took marks

1. impression dimension

2. patterns (ridges, striation)

3. defects (nicks, chips)

4. trace evidence left on tool

What is a forensic odontologist?

Field that combines trained dentist skills with law enforcement skills

2 types of cases forensic odontologist work on?

1. identifying human remains

2. bite mark matching

What is an odontogram?

a person’s dental chart

Why are odontogram used for identification before fingerprinting?

more people have their teeth charted at their dentist compared to having their fingerprint taken

6 characteristics that are used to match an odontogram to human remains

1. tooth loss

2. width of the bite

3. spacing between teeth

4. filling

5. crowns

6. braces

What did PCAST conclude about forensic bite-mark evidence?

bite-mark evidence is not scientifically valid

3 steps used to match bite mark found at crime scene to a suspect

1. photograph next to scale

2. replica is made

3. attempting a match

5 problems with matching bite marks to a suspect

1. marks are ill defined or distorted

2. full marks are rarely recovered

3. bites leave bruising

4. bite marks can be distorted by victims motion

5. victim’s position and skin physiology can affect details and shape

Information that can be learned by looking at marks. on a cartridge case

location size, shape of the firing pin, size of extractors or ejectors

How are striations made on fired bullets?

when the bullet travels through barrel interior as it’s fired

What is rifling?

the groove pattern on the internal surface of a gun barrel

Why do handguns have rifling?

creates a spin to the bullet

4 measures of a gun’s rifling

1. number of lands and grooves

2. diameter of lands and grooves

3. width of lands and grooves

4. depth of grooves

3 reasons why can bullet striations be used to identify marks?

1. no two firearms produce the same unique striation

2. firearms don’t normally change over time

3. firearm’s surface characteristics cannot be exactly reproduced in other firearms

What do forensic scientists do with a suspect gun?

they test fire it to obtain bullets and cartridges for comparison

National Integrated Ballistic Information Network (NIBN)

contained all digital images of recovered/tested fired ammunition markings

DRUGFIRE

database of firearms evidence from serial shooting investigations

Parts of ammunition (top to bottom)

bullet, case, charge, primer